I intend to analyze the effects of fixative composition on the numbers of synaptic vesicles seen in nerve terminals. Fast-frozen, freeze substituted tissues will be used as a standard against which to measure the effects of fixatives containing various cations. These studies will be used as a guide in choosing fixation conditions to use in attempting to correlate vesicle turnover and quantal transmitter release. Studies of fixation conditions required to produce an electron dense structure in electric organ cholinergic synaptic vesicles will be carried out with the intention of learning how to observe this phenomenon in mammalian tissues.